Use mapnik scale-independent pixel size expressions

!pixel_height! and !pixel_width! do not work for area-based filtering
when using high dpi rendering. Instead, !scale_denominator! needs to
be used.

project.mml

@@ -74,7 +74,7 @@ Layer:
             FROM planet_osm_polygon
             WHERE (landuse IN ('forest', 'farmland', 'residential', 'commercial', 'retail', 'industrial', 'meadow', 'grass', 'village_green', 'vineyard', 'orchard')
               OR "natural" IN ('wood', 'wetland', 'mud', 'sand', 'scree', 'shingle', 'bare_rock', 'heath', 'grassland', 'scrub'))
-              AND way_area > 0.01*!pixel_width!::real*!pixel_height!::real
+              AND way_area > 0.01*POW(!scale_denominator!*0.001*0.28,2)
               AND building IS NULL
           ) AS features
           ORDER BY way_area DESC, feature
@@ -133,7 +133,7 @@ Layer:
               OR tourism IN ('camp_site', 'caravan_site', 'picnic_site')
               OR highway IN ('services', 'rest_area')
               OR railway = 'station')
-              AND way_area > 1*!pixel_width!::real*!pixel_height!::real
+              AND way_area > 1*POW(!scale_denominator!*0.001*0.28,2)
           ) AS landcover
           ORDER BY way_area DESC, feature
         ) AS features
@@ -241,7 +241,7 @@ Layer:
               OR landuse IN ('reservoir', 'basin')
               OR "natural" IN ('water', 'glacier'))
             AND building IS NULL
-            AND way_area > 1*!pixel_width!::real*!pixel_height!::real
+            AND way_area > 1*POW(!scale_denominator!*0.001*0.28,2)
           ORDER BY COALESCE(layer,0), way_area DESC
         ) AS water_areas
     properties:
@@ -287,7 +287,7 @@ Layer:
           FROM planet_osm_polygon
           WHERE ("natural" IN ('mud', 'wetland', 'wood', 'beach', 'shoal', 'reef', 'scrub', 'sand') OR landuse = 'forest')
             AND building IS NULL
-            AND way_area > 1*!pixel_width!::real*!pixel_height!::real
+            AND way_area > 1*POW(!scale_denominator!*0.001*0.28,2)
           ORDER BY COALESCE(layer,0), way_area DESC
         ) AS landcover_area_symbols
     properties:
@@ -437,7 +437,7 @@ Layer:
           FROM planet_osm_polygon
           WHERE building IS NOT NULL
             AND building != 'no'
-            AND way_area > 1*!pixel_width!::real*!pixel_height!::real
+            AND way_area > 1*POW(!scale_denominator!*0.001*0.28,2)
           ORDER BY COALESCE(layer,0), way_area DESC
         ) AS buildings
     properties:
@@ -1145,7 +1145,7 @@ Layer:
                  OR leisure = 'nature_reserve'
                  OR (boundary = 'protected_area' AND tags->'protect_class' IN ('1','1a','1b','2','3','4','5','6','7','24','97','98','99')))
             AND building IS NULL
-            AND way_area > 1*!pixel_width!::real*!pixel_height!::real
+            AND way_area > 1*POW(!scale_denominator!*0.001*0.28,2)
         ) AS protected_areas
     properties:
       cache-features: true
@@ -1184,7 +1184,7 @@ Layer:
             AND boundary = 'administrative'
             AND admin_level = '2'
             AND name IS NOT NULL
-            AND way_area > 100*!pixel_width!::real*!pixel_height!::real
+            AND way_area > 100*POW(!scale_denominator!*0.001*0.28,2)
             AND osm_id < 0
           ORDER BY way_area DESC
         ) AS country_names
@@ -1228,7 +1228,7 @@ Layer:
             AND boundary = 'administrative'
             AND admin_level = '4'
             AND name IS NOT NULL
-            AND way_area > 100*!pixel_width!::real*!pixel_height!::real
+            AND way_area > 100*POW(!scale_denominator!*0.001*0.28,2)
             AND osm_id < 0
           ORDER BY way_area DESC
         ) AS state_names